Laminating encapsulant film containing phosphor over LEDs

ABSTRACT

A process is described for wavelength conversion of LED light using phosphors. LED dies are tested for correlated color temperature (CCT), and binned according to their color emission. The LEDs in each_bin are mounted on a single submount to form an array of LEDs. Various thin sheets of a flexible encapsulant (e.g., silicone) infused with one or more phosphors are preformed, where each sheet has different color conversion properties. An appropriate sheet is placed over an array of LED mounted on a submount, and the LEDs are energized. The resulting light is measured for CCT. If the CCT is acceptable, the phosphor sheet is permanently laminated onto the LEDs and submount. By selecting a different phosphor sheet for each bin of LEDs, the resulting CCT is very uniform across all bins.

This is a continuation of prior application Ser. No. 12/050,132 filedMar. 12, 2008 which is a continuation of granted U.S. Pat. No. 7,344,952filed Oct. 28, 2005 both of which are incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates to light emitting diodes (LEDs) and, inparticular, to wavelength conversion of the light from LED chips.

BACKGROUND

Light emitting diodes (LEDs) can be fabricated to generate light indifferent colors. For producing white light, various colors can becombined. One way of producing white light from an LED structure is todeposit a yellow phosphor over a blue LED. There are many ways to dothis. One of the most common ways is to mix phosphor particles in anencapsulation material and coat it over the LED. Another way iselectrophoretic deposition where phosphor particles are depositeddirectly onto LEDs.

White light is produced when some of the blue light that is allowed toleak through the phosphor layer combines with the yellow light.

Another way of producing white light is to combine red, green, and bluelight. This can be achieved either through the use of red, green andblue LEDs or through blue LEDs with red and green phosphors.

LEDs, such as blue and UV GaN LEDs, vary slightly in color emission evenin the same wafer. Applying the same phosphor coating to all LEDs willthus result in a range of color temperatures (i.e., correlated colortemperature or CCT) due to the varying characteristics of the LEDs.Further, variations in the amount of phosphor in the coating will alsoundesirably increase the CCT range. As a result, the output color (e.g.,CCT of white light) can vary greatly, even though the fabricationprocess is the same.

Known or common challenges with some of the above methods include: 1)complexity of the phosphor deposition process; and 2) variations in theamount of phosphor from LED to LED, resulting in inconsistent colortemperatures.

SUMMARY

A highly reliable and simple process is described for wavelengthconversion of LED light using phosphors. The process results in auniform color temperature across all LEDs despite the LEDs emittinglight with varying color temperatures.

LED dies are formed on wafers, tested, separated, and binned accordingto their color emission. In one example, the LEDs are GaN based blueLEDs with an emission of about 450 nm, where the range of wavelengths ineach bin can be, for example, 5 nm. In one example, there are four bins.

The LEDs in a single bin are mounted on a single submount to form anarray of LEDs. In one example, there are around 400 LEDs mounted on asingle submount. In one example, the submount is a rigid substrate thatprovides top contact pads for mounting each LED, where the top pads areelectrically connected to bottom contact pads for connection to aprinted circuit board or other structure. The submount may also containmetal traces that interconnect multiple LEDs together.

Various thin sheets (e.g., 100 microns thick) of a flexible encapsulant(e.g., silicone) infused with one or more types of phosphors arepreformed. There may be one or more types of sheets associated with eachbin of LEDs. Each sheet has different color conversion properties. Thecolor conversion properties may be varied by varying the phosphorconcentration in the encapsulant within a range of, for example, 20-70weight %. The thickness of the sheet may also be varied from, forexample, 50-200 microns to vary the amount of phosphor in the sheet. Thesheets may also be varied by varying both the thickness and the weightratio.

The characteristics of each sheet may be determined empirically byenergizing each sheet with the light emitted by a binned LED andadjusting the sheet until the resulting light meets the desired colorcriteria.

After the phosphor sheets are formed, the appropriate sheet is placedover an array of LEDs mounted on a submount, and the LEDs are energizedby probing LED contact pads or applying a current to any test pads onthe periphery of the submount. The resulting light is then measured forCCT. If the CCT is acceptable, the phosphor sheet is permanentlylaminated onto the LEDs and submount such as by heat and pressure or bya combination of heat and a vacuum. If the tested CCT was not adequate,a different type of phosphor sheet would be used and tested.

The lamination encapsulates each LED to protect the LEDs fromcontaminants and damage.

Since there is a range of colors in a bin, there may be multiplephosphor sheets associated with each bin.

After the lamination, the LEDs in the array of LEDs on the submount areseparated. The separated and encapsulated LEDs may then be mounted onPCBs.

In another embodiment, the phosphor sheets are associated with bins, andthere is no further test to determine whether the particular sheetproduces the desired CCT. It is assumed the CCT is correct.

Since the phosphor sheets are of a uniform thickness and the differentsheets compensate for the variations in wavelength emitted by the LEDs,the resulting CCT is very uniform.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top down view of a wafer on which is formed LEDs.

FIG. 2 illustrates how the LED separated from the wafer are tested andbinned according to their peak emission wavelength.

FIG. 3A is a front perspective view of LEDs of a single bin mounted on asubmount. The submount shows optional test pads along its peripheryelectrically connected to the LEDs.

FIG. 3B is a back perspective view of the submount of FIG. 3A showingthe back contact pads of the submount connected to the n and p metal ofone or more associated LEDs.

FIG. 4 illustrates various steps in one embodiment of the inventiveprocess for laminating one of a plurality of phosphor sheets to an arrayof LEDs on a submount to achieve a uniform output CCT.

FIG. 5 is a flowchart describing one embodiment of the inventiveprocess.

DETAILED DESCRIPTION

FIG. 5 is a flowchart illustrating one embodiment of a process forwavelength conversion of LED light to produce a uniform output CCTdespite the LEDs emitting light with different color temperatures.

In step 10 of FIG. 5, LEDs are formed on a wafer in a conventionalmanner. FIG. 1 is a front view of LEDs 12 formed on a growth wafer 14.In one embodiment, the growth wafer is sapphire, GaN, or SiC, and theLEDs are GaN-based LEDs that emit a blue or UV light. In otherembodiments, the LEDs and growth substrate may be any type. GaN basedLEDs are particularly suitable for forming blue or UV LEDs frequentlyused in conjunction with a phosphor for wavelength conversion.

In step 16 of FIG. 5, the individual LEDs are probed to energize eachLED. The peak wavelength of emitted light from each LED is measured by acolor detector. In the present example, the LEDs' measured color fallsinto one of four ranges of color. Each range is referred to as a bin. Inone example, as shown in FIG. 2, the LEDs emit blue light and aregrouped in one of four bins 1-4 as follows: 1) 440-445 nm; 2) 445-450nm; 3) 450-455 nm; and 4) 455-460 nm. There may be additional bins ordifferent criteria for each bin. The bin number for each LED istemporarily stored in a memory.

In step 18, the LEDs 12 are separated such as by scribing and breakingIn one embodiment, the wafer is first placed on a stretchable adhesivesheet. After scribing, a roller bends the wafer to break along thescribe lines. The sheet is then stretched to physically separate theLEDs from each other. The LEDs are then removed from the adhesive sheetand grouped (binned) according to their preassigned bins. All the aboveprocesses may be performed automatically by programmed machines.

In step 20, the LEDs in a single bin are mounted on a submount. Thesubmount may be a ceramic, silicon, or other material substrate withelectrical interconnections formed on its surface. FIG. 3A is frontperspective view of LEDs 12 mounted on submount 24. In one embodiment,there are about 400 LEDs mounted on a single submount. The LEDs 12 haven and p metal pads, and the top surface of submount 24 has correspondingmetal pads. The LED pads may be directly bonded to the submount pads byany means such as solder or ultrasonic weld. Alternatively, one or moreof the LED pads may be connected to the submount pads by a wireconnector.

The submount pads are electrically connected, such as by metal tracesand vias, to other submount contact pads for connection to a powersupply. For example, the submount's top contact pads may be connected tocorresponding n and p bottom contact pads 26 shown in the backperspective view of FIG. 3B. The back contact pads 26 may later bebonded to corresponding pads on a printed circuit board or otherstructure.

FIG. 3A additionally shows test pads 30 around the periphery of submount24. The test pads 30 are electrically connected to the various topcontact pads for energizing groups or all of the LEDs at the same time.

In a parallel process, phosphor sheets are formed. In step 40 of FIG. 5,a suitable phosphor powder is mixed with a liquid encapsulant materialsuch as silicone, epoxy, an acrylic, or other suitable material.Phosphor powders that convert blue or UV light to white light are wellknown. A phosphor that converts blue light to yellow light may be usedfor creating white light by allowing some blue light to leak through thephosphor layer. A red phosphor may be combined with the yellow phosphorto add warmth to the white light. Alternatively, the phosphor powder mayinclude green and red phosphors for adding red and green components tothe blue component to create white light. If a UV LED is used, a bluephosphor would additionally be used in the phosphor sheet.

Thin phosphor sheets may be formed by depositing a silicone-phosphormixture on a non-stick release layer overlying a flat surface and thenspinning the material to form a thin sheet of uniform thickness. A thinsheet may also be formed by depositing a predetermined amount of thesilicone-phosphor mixture in a mold, or by spraying a predeterminedamount of material on a release layer.

The amount of phosphor that resides over each portion of the LEDdetermines the percentage of LED light that is wavelength-converted bythe phosphor. The amount of phosphor over each portion of the LED may beadjusted by forming phosphor sheets with different densities of phosphor(i.e., different phosphor-to-encapsulant weight ratios), or withdifferent ratios of two or more phosphor (e.g., red and green) powders,or with different thicknesses of the sheets, or using a combination ofthese. In one embodiment, there is a phosphor sheet associated with eachof the four bins with each sheet having a differentphosphor-to-encapsulant weight ratio. The ratios will typically be inthe range of 20-70%. If the thickness of the sheets are to be varied toachieve the desired wavelength conversion, the thickness may rangebetween 50-200 microns.

In one embodiment, the phosphor sheet is partially cured by applyingheat so that the sheet is flexible and somewhat tacky. The sheet is thenremoved from the flat surface along with the release layer.

The particular characteristics of each sheet may be determinedempirically by subjecting the sheet to light of a certain wavelength andthen measuring the resulting emission. If the emission does not meet therequired CCT, the characteristics of the sheet are adjusted (step 42)by, for example, changing the weight ratio of the phosphor(s) and/orchanging the thickness of the sheet. Once the sheet formulas for eachbinned LED are satisfactory, quantities of the sheets may be made.

In step 44 of FIG. 5, each type of phosphor sheet is associated with abin number to achieve a consistent CCT of the output light across allbins.

FIG. 4 illustrates the process for laminating a selected phosphor sheetonto the LEDs mounted on a submount. Side views of the submount 24substrate, LEDs 12, and phosphor sheets 51, 52, 53, and 54 are shown. Inone example, sheet 51 is associated with bin 1, sheet 52 is associatedwith bin 2, sheet 53 is associated with bin 3, and sheet 54 isassociated with bin 4.

In step 46 of FIG. 5, the phosphor sheet 51 associated with bin 1 isplaced over the array of LEDs 12 from bin 1 on the submount 24.

In step 48, the LEDs are then energized by probing or by using test padson the submount. Ideally, all or large groups of LEDs are energized atthe same time to save time. Alternatively, only one or a few sample LEDsmay be energize to determine if the sheet 51 is suitable.

In step 49, the resulting light is measured using a color detector andcompared to a predetermined CCT (e.g., a particular white point). Instep 58, if the CCT meets the desired criteria, the sheet is deemedacceptable. In step 59, if the sheet 51 does not produce the desiredcolor, a different sheet (either chosen from sheets 52-54 or additionalsheets) is placed over the LED array, and testing is repeated. In oneembodiment, two or three different sheets are associated with each bin,and the best fit sheet is used for the array.

In step 60, the selected sheet is laminated onto the LED array by heatand pressure. This encapsulates each LED to protect it from contaminantsand provides a uniform thickness phosphor layer over the LED. In oneembodiment, a soft (e.g., rubber) pad is applied to the sheet 51 underpressure while the area is heated to cure the encapsulant. Curing mayalso be by UV. The phosphor sheet may also be laminated by providing avacuum between the sheet and the submount. Once cured, the encapsulantadheres to the LED and submount. The encapsulant may cure hard or may berelatively soft. Any release layer film is then removed from the top ofthe phosphor sheet.

In another embodiment, the phosphor sheet is formed over a thin adhesivelayer, and the adhesive layer (along with the phosphor sheet) is affixedto the LEDs and submount. The adhesive is then cured. The adhesive layermay be any type of transparent layer such as epoxy.

FIG. 4 shows the phosphor sheet 51 conforming to the shape of the LEDsafter encapsulation.

In step 62, after curing, the LEDs on the submount may be separated byscribing and breaking, sawing, or other technique. The singulated,encapsulated LEDs form individual packaged LEDs. The LEDs may beadditionally packaged to provide lenses, terminal pins for soldering,heat sinks, and a protective structure.

In another embodiment, there is no separate testing in steps 48, 49, 58,and 59 since the sheets have already been tested when formulating thesheets for each bin. It is assumed the sheets preassociated with thebins will perform the desired wavelength conversion.

Having described the invention in detail, those skilled in the art willappreciate that, given the present disclosure, modifications may be madeto the invention without departing from the spirit of the inventiveconcept described herein. Therefore, it is not intended that the scopeof the invention be limited to the specific embodiments illustrated anddescribed.

What is claimed is:
 1. A method of fabricating a light emitting diode(LED) comprising: separating LEDs produced to provide a common nominalpeak wavelength, into a plurality of groups of LEDs, each group of LEDsproducing a different range of peak wavelengths about the common nominalwavelength, determined by testing; mounting a plurality of LEDs of asingle particular group onto a submount; providing a plurality ofdifferent wavelength-conversion sheets, each sheet comprising a phosphorcombined with a flexible encapsulate material, each sheet having adifferent phosphor conversion characteristic; selecting a particular oneof the wavelength-conversion sheets for applying over the LEDs on thesubmount; energizing one or more LEDs on the submount; detecting anemitted color generated by the combination of the energized one or moreLEDs and the particular wavelength-conversion sheet to ensure theemitted color meets a predetermined color; and encapsulating the LEDsfrom the single particular group mounted on the submount with theparticular wavelength-conversion sheet if the emitted color meets thepredetermined color.
 2. The method of claim 1 wherein each LED within agroup of LEDs generates blue light.
 3. The method of claim 1 whereineach LED within a group of LEDs generates UV light.
 4. The method ofclaim 1 further comprising selecting a different wavelength-conversionsheet if, during the step of detecting an emitted color, the detectedcolor does not match the predetermined color.
 5. The method of claim 1wherein the predetermined color is a particular color temperature forwhite.
 6. The method of claim 1 wherein there is one or more differentwavelength-conversion sheets for each group of LEDs.
 7. The method ofclaim 1 wherein the range of peak wavelengths in each group is betweenabout 3 and 10 nm.
 8. The method of claim 1 wherein the differentwavelength-conversion sheets differ by different weight ratios ofphosphor to encapsulant or different thicknesses of the sheets.
 9. Themethod of claim 1 wherein a thickness of each wavelength-conversionsheet is between about 50-200 microns.
 10. The method of claim 1 whereinthere are different types of phosphors in a wavelength-conversion sheet.11. The method of claim 1 wherein encapsulating the LEDs comprisesheating the selected wavelength-conversion sheet while pressing it overthe LEDs.
 12. The method of claim 1 wherein encapsulating the LEDscomprises heating the selected wavelength-conversion sheet while overthe LEDs and providing a vacuum between the sheet and the submount. 13.The method of claim 1 wherein there are multiple wavelength-conversionsheets associated with each group of LEDs.
 14. The method of claim 1wherein encapsulating the LEDs results in the predetermined color beingsubstantially achieved for all groups of LEDs by combining any group ofLEDs with a particular one of the different wavelength-conversionsheets.
 15. The method claim 1 further comprising separatingencapsulated LEDs from other LEDs mounted on the submount.
 16. Themethod of claim 1 wherein the submount provides electrical contacts forenergizing the LEDs on the submount.
 17. The method of claim 1 whereinproviding a plurality of different wavelength-conversion sheetscomprises depositing a liquid encapsulant containing a phosphor on aflat surface and spinning the flat surface until the encapsulant is of adesired thickness.
 18. The method of claim 1 wherein providing aplurality of different wavelength-conversion sheets comprises depositinga liquid encapsulant containing a phosphor on a flat surface in a moldsuch that the encapsulant is of a desired thickness.
 19. The method ofclaim 1 wherein the LEDs are GaN based LEDs.
 20. The method of claim 1wherein the phosphor generates yellow light.
 21. The method of claim 1wherein the phosphor comprises a plurality of phosphor materials andgenerates red and green light.
 22. The method of claim 1 wherein thesteps of energizing and detecting are performed only during formulationof the plurality of wavelength-conversion sheets and when determiningwhich particular one of the wavelength-conversion sheets is to beassociated with each group of LEDs.
 23. The method of claim 1, whereinthe LEDs are produced on a single die.
 24. A method of fabricating alight emitting diode (LED) comprising: separating LEDs produced toprovide a common nominal peak wavelength into a plurality of groups ofLEDs, each group of LEDs producing a different range of peak wavelengthsabout the common nominal wavelength, determined by testing; providing aplurality of groups of wavelength-conversion sheets, each group ofsheets being configured to provide a predetermined color when combinedwith LEDs of a corresponding group of LEDs; mounting a plurality of LEDsof a single particular group of LEDs onto a submount; and encapsulatingthe LEDs mounted on the submount with a wavelength-conversion sheetselected from the group of sheets corresponding to the particular groupof LEDs.
 25. The method of claim 24 wherein the predetermined color is aparticular color temperature for white.
 26. The method of claim 24wherein the range of peak wavelengths in each group of LEDs is betweenabout 3 and 10 nm.
 27. The method of claim 24 wherein the differentgroups of wavelength-conversion sheets differ by different weight ratiosof phosphor to encapsulant.
 28. The method of claim 24 wherein thedifferent groups of wavelength-conversion sheets differ by differentthicknesses of the sheets.
 29. The method claim 24 further comprisingseparating encapsulated LEDs from other LEDs mounted on the submount.30. The method of claim 24, wherein the LEDs produced are produced on asingle die.